This invention relates to a direct drive electronic ballast circuit especially suitable for a fluorescent lamp load and more particularly to a direct drive ballast circuit having an oscillator inverter with a drive circuit dependent upon load current and initially responsive to energization from an oscillator starting circuit.
Presently, fluorescent lamp systems commonly employ ballast circuitry of the auto-transformer type. Such apparatus is obviously undesirably heavy, cumbersome and expensive as compared with ballast circuitry of the electronic type. Also, the auto-transformer type ballast is relatively inefficient, generates undesired heat, wastes energy and operates in the audible range.
Another popular form of ballast circuitry includes a flip-flop oscillator and saturable core transformer. The transformer core saturation characteristics are employed to limit current flow. Unfortunately, control and predictability of such current flow is difficult and unreliable in such apparatus as compared with electronic ballast circuitry.
A further form of ballast circuit, set forth in the previously mentioned copending application bearing U.S. Pat. No. 4,109,307, provides a storage capability whereby the pulsed DC potential applied to a tuned oscillator is altered to provide application of a substantially constant DC potential. However, it has been found that such circuitry leaves something to be desired with regard to unexpected changes in loading.
Still another form of electronic ballast circuitry provides a high frequency inverter coupled to a source of pulsed DC potential shunted by an energy storage and isolating circuit. Also, a feedback rectifier supplies energy to the storage and isolating circuit while the inverter is driven in accordance with current flow in a load circuit. Moreover, a relatively complex drive system is utilized to provide base drive for the high frequency inverter.